Process control valves are used in systems or processes that require regulation or control of process fluid flows. Such control valves may be configured as sliding stem valves, rotary valves, globe valves, etc. By setting a position of a control element, such as a valve plug, relative to a valve seat, fluid flow through the control valve may be precisely set. The valve plug slides or moves relative to the valve seat generally within a valve body. In some control valves, a cage may be included, the valve plug sliding within the cage. Because the valve plug slides within the valve body, preventing fluid flow between the valve plug and the valve body (or cage) is important. At the same time, reducing friction and minimizing wear and adhesion between the valve plug and the valve body (or cage) is desirable.
In some control valves, a seal may be incorporated into valve plug itself, or the seal may be incorporated into the valve body (or cage) or bonnet. Regardless, the seal must prevent fluid from flowing between the valve plug and the valve body or bonnet. In an effort to both prevent fluid from flowing between the control element and the valve body or bonnet, and to reduce friction generated between the control element and valve body or bonnet, some sliding stem valves have incorporated relatively soft materials into the seal. Such seals are spring loaded, pressure assisted lip seals constructed from neat, filled, or reinforced polytetrafluoroethylene jackets with a corrosion resistant spring constructed from stainless steel, nickel or cobalt based alloys. Other fluoropolymers, ultrahigh molecular polyethylene, and polyurethane are other jacket materials that are used. Other types of seals that have been used include synthetic o-rings, quad-rings, backup rings with PTFE based wear rings and metal reinforced PTFE based monolithic seals. While reducing friction and providing a high quality, resilient seal, soft materials are generally not useful in high temperature environments because the material may melt, creep, or otherwise degrade under high temperature conditions. All of these soft materials are only used for relatively low temperatures, less than 600° F. (less than about 316° C.).
Conventional high temperature sliding stem control valves have typically incorporated monolithic, hard graphite seal rings that can withstand high temperatures. While the graphite seal rings are able to withstand high temperatures, the graphite seal rings do not generally provide an optimum seal between the valve plug and the valve body (or cage) or bonnet because they are not flexible, compliant, or resilient. Graphite seal rings may be damaged from friction created between the valve plug and the valve body (or cage) and they are relatively vulnerable to particles and contaminants in the flow stream particles or contaminants that have adhered to the valve plug or valve body (or cage). The lack of compliance in monolithic graphite materials results in significantly more shutoff leakage than the softer rubber or plastic compliant materials that are employed in low temperature environments. Additionally, graphite seal rings have relatively high coefficients of friction, which leads to excess friction and wear when the valve plug is moved. This excess friction and wear leads to degradation and early failure of the seal and/or the control element.